Myxococcus xanthus, a gram-negative bacterium, contains a transmembrane protein serine/threonine kinase that blocks the secretion of beta-lactamase by phosphorylation

The expanding world of protein kinase-like families in bacteria: forty families and counting

The protein kinase-like clan/superfamily is a large group of regulatory, signaling and biosynthetic enzymes that were historically regarded as typically eukaryotic proteins, although bacterial members have also been known for a long time. In this review, we explore the diversity of bacterial protein kinase like families, and discuss functional versatility of these enzymes, both the ones acting within the bacterial cell, and those acting within eukaryotic cells as effectors during infection. We focus on novel bacterial kinase-like families discovered in the last five years. A bioinformatics perspective is held here, hence sequence and structure comparison overview is presented, and also a comparison of genomic neighbourhoods of the families. We perform a phylum-level census of the families. Also, we discuss apparent pseudokinases that turned out to perform alternative catalytic functions by repurposing their atypical kinase-like active sites. We also highlight some 'unpopular' kinase-like families that await characterisation.

Dynamical patterning modules and network motifs as joint determinants of development: Lessons from an aggregative bacterium

Development and evolution are dynamical processes under the continuous control of organismic and environmental factors. Generic physical processes, associated with biological materials and certain genes or molecules, provide a morphological template for the evolution and development of organism forms. Generic dynamical behaviors, associated with recurring network motifs, provide a temporal template for the regulation and coordination of biological processes. The role of generic physical processes and their associated molecules in development is the topic of the dynamical patterning module (DPM) framework. The role of generic dynamical behaviors in biological regulation is studied via the identification of the associated network motifs (NMs). We propose a joint DPM-NM perspective on the emergence and regulation of multicellularity focusing on a multicellular aggregative bacterium, Myxococcus xanthus. Understanding M. xanthus development as a dynamical process embedded in a physical substrate provides novel insights into the interaction between developmental regulatory networks and generic physical processes in the evolutionary transition to multicellularity.

The Eukaryote-Like Serine/Threonine Kinase STK Regulates the Growth and Metabolism of Zoonotic Streptococcus suis

Like eukaryotes, bacteria express one or more serine/threonine kinases (STKs) that initiate diverse signaling networks. The STK from Streptococcus suis is encoded by a single-copy stk gene, which is crucial in stress response and virulence. To further understand the regulatory mechanism of STK in S. suis, a stk deletion strain (Δstk) and its complementary strain (CΔstk) were constructed to systematically decode STK characteristics by applying whole transcriptome RNA sequencing (RNA-Seq) and phosphoproteomic analysis. Numerous genes were differentially expressed in Δstk compared with the wild-type parental strain SC-19, including 320 up-regulated and 219 down-regulated genes. Particularly, 32 virulence-associated genes (VAGs) were significantly down-regulated in Δstk. Seven metabolic pathways relevant to bacterial central metabolism and translation are significantly repressed in Δstk. Phosphoproteomic analysis further identified 12 phosphoproteins that exhibit differential phosphorylation in Δstk. These proteins are associated with cell growth and division, glycolysis, and translation. Consistently, phenotypic assays confirmed that the Δstk strain displayed deficient growth and attenuated pathogenicity. Thus, STK is a central regulator that plays an important role in cell growth and division, as well as S. suis metabolism.

Comparative Phosphoproteomics Reveals the Role of AmpC β-lactamase Phosphorylation in the Clinical Imipenem-resistant Strain Acinetobacter baumannii SK17

Nosocomial infectious outbreaks caused by multidrug-resistant Acinetobacter baumannii have emerged as a serious threat to human health. Phosphoproteomics of pathogenic bacteria has been used to identify the mechanisms of bacterial virulence and antimicrobial resistance. In this study, we used a shotgun strategy combined with high-accuracy mass spectrometry to analyze the phosphoproteomics of the imipenem-susceptible strain SK17-S and -resistant strain SK17-R. We identified 410 phosphosites on 248 unique phosphoproteins in SK17-S and 285 phosphosites on 211 unique phosphoproteins in SK17-R. The distributions of the Ser/Thr/Tyr/Asp/His phosphosites in SK17-S and SK17-R were 47.0%/27.6%/12.4%/8.0%/4.9% versus 41.4%/29.5%/17.5%/6.7%/4.9%, respectively. The Ser-90 phosphosite, located on the catalytic motif S(88)VS(90)K of the AmpC β-lactamase, was first identified in SK17-S. Based on site-directed mutagenesis, the nonphosphorylatable mutant S90A was found to be more resistant to imipenem, whereas the phosphorylation-simulated mutant S90D was sensitive to imipenem. Additionally, the S90A mutant protein exhibited higher β-lactamase activity and conferred greater bacterial protection against imipenem in SK17-S compared with the wild-type. In sum, our results revealed that in A. baumannii, Ser-90 phosphorylation of AmpC negatively regulates both β-lactamase activity and the ability to counteract the antibiotic effects of imipenem. These findings highlight the impact of phosphorylation-mediated regulation in antibiotic-resistant bacteria on future drug design and new therapies.

Eukaryote-like serine/threonine kinases and phosphatases in bacteria

Genomic studies have revealed the presence of Ser/Thr kinases and phosphatases in many bacterial species, although their physiological roles have largely been unclear. Here we review bacterial Ser/Thr kinases (eSTKs) that show homology in their catalytic domains to eukaryotic Ser/Thr kinases and their partner phosphatases (eSTPs) that are homologous to eukaryotic phosphatases. We first discuss insights into the enzymatic mechanism of eSTK activation derived from structural studies on both the ligand-binding and catalytic domains. We then turn our attention to the identified substrates of eSTKs and eSTPs for a number of species and to the implications of these findings for understanding their physiological roles in these organisms.

A framework for classification of prokaryotic protein kinases

BACKGROUND

Overwhelming majority of the Serine/Threonine protein kinases identified by gleaning archaeal and eubacterial genomes could not be classified into any of the well known Hanks and Hunter subfamilies of protein kinases. This is owing to the development of Hanks and Hunter classification scheme based on eukaryotic protein kinases which are highly divergent from their prokaryotic homologues. A large dataset of prokaryotic Serine/Threonine protein kinases recognized from genomes of prokaryotes have been used to develop a classification framework for prokaryotic Ser/Thr protein kinases.

METHODOLOGY/PRINCIPAL FINDINGS

We have used traditional sequence alignment and phylogenetic approaches and clustered the prokaryotic kinases which represent 72 subfamilies with at least 4 members in each. Such a clustering enables classification of prokaryotic Ser/Thr kinases and it can be used as a framework to classify newly identified prokaryotic Ser/Thr kinases. After series of searches in a comprehensive sequence database we recognized that 38 subfamilies of prokaryotic protein kinases are associated to a specific taxonomic level. For example 4, 6 and 3 subfamilies have been identified that are currently specific to phylum proteobacteria, cyanobacteria and actinobacteria respectively. Similarly subfamilies which are specific to an order, sub-order, class, family and genus have also been identified. In addition to these, we also identify organism-diverse subfamilies. Members of these clusters are from organisms of different taxonomic levels, such as archaea, bacteria, eukaryotes and viruses.

CONCLUSION/SIGNIFICANCE

Interestingly, occurrence of several taxonomic level specific subfamilies of prokaryotic kinases contrasts with classification of eukaryotic protein kinases in which most of the popular subfamilies of eukaryotic protein kinases occur diversely in several eukaryotes. Many prokaryotic Ser/Thr kinases exhibit a wide variety of modular organization which indicates a degree of complexity and protein-protein interactions in the signaling pathways in these microbes.

TagR promotes PpkA-catalysed type VI secretion activation in Pseudomonas aeruginosa

Type VI secretion systems (T6SSs) contribute to interactions of bacterial pathogens and symbionts with their hosts. Previously, we showed that Pseudomonas aeruginosa T6S is posttranslationally activated upon phosphorylation of Fha1, an FHA domain protein, by PpkA, a membrane-spanning threonine kinase. Herein, additional structural, enzymatic and genetic requirements for PpkA-catalysed T6SS activation are identified. We found that PpkA plays an essential structural role in the T6SS, and that this role is intimately linked to its ability to promote secretion and phosphorylate Fha1. Protein localization and protein-protein interaction studies show that a complex containing Fha1 and the T6S ATPase, ClpV1 is recruited to the T6S apparatus in a phosphorylation-dependent manner. The mechanism of PpkA activation was also investigated. We identified critical PpkA autophosphorylation sites and showed that small molecule-induced dimerization of the extracellular domains of PpkA is sufficient to activate the T6SS. Finally, we discovered TagR, a component of the T6S posttranslational regulatory pathway that functions upstream of PpkA to promote kinase activity. We present a model whereby an unknown cue causes dimerization of the extracellular domains of PpkA, leading to its autophosphorylation, recruitment of the Fha1-ClpV1 complex, phosphorylation of Fha1, and T6SS activation. Our findings should facilitate approaches for identifying physiological activators of T6S.

A novel regulation on developmental gene expression of fruiting body formation in Myxobacteria

Myxobacteria are Gram-negative soil microorganisms that prey on other microorganisms. Myxobacteria have significant potential for applications in biotechnology because of their extraordinary ability to produce natural products such as secondary metabolites. Myxobacteria also stand out as model organisms for the study of cell-cell interactions and multicellular development during their complex life cycle. Cellular morphogenesis during multicellular development in myxobacteria is very similar to that in the eukaryotic soil amoebae. Recent studies have started uncovering molecular mechanisms directing the myxobacterial life cycle. We describe recent studies on signal transduction and gene expression during multicellular development in the myxobacterium Myxococcus xanthus. We provide our current model for signal transduction pathways mediated by a two-component His-Asp phosphorelay system and a Ser/Thr kinase cascade.

Factors that modulate the Pkn4 kinase cascade in Myxococcus xanthus

Myxococcus xanthus, a gram-negative developmental bacterium, contains a large number of protein Ser/Thr kinases (PSTKs). Among these PSTKs, Pkn4 is shown to be 6-phosphofructokinase (PFK) kinase. PFK associates with the regulatory domain of Pkn4 (Pkn4RD) and is activated 2.7-fold upon phosphorylation at Thr-226 by Pkn4. The activation of PFK is required to consume glycogen accumulated during early development and is essential for efficient sporulation. Three new factors, MkapA, MkapB and MkapC have been identified that associate with Pkn4 by the yeast two-hybrid screen and each contains well-known protein-protein interaction domains. MkapB interacts with Pkn4 in a phosphorylation-dependent manner and remains associated with Pkn4 after its phosphorylation. Binding of MkapB to Pkn4 prevents the interaction of Pkn4 with PFK and consequently PFK phosphorylation and activation. A pfk-pkn4 deletion mutant accumulates glycogen at a rate two folds higher than the parent strain, DZF1, at the stationary phase and early development stage, it is unable to consume glycogen during development and produces only 3.4% of the DZF1 spore yield. In contrast, an mkapB deletion mutant exhibits a 24 h delay in fruiting body formation, accumulates less glycogen in the stationary phase and gives rise to 6.4% of the DZF1 spore yield. In addition to Pkn4, MkapA associates with other membrane-associated PSTKs, Pkn1, Pkn2, Pkn8 and Pkn9, while MkapB associates with Pkn8 and Pkn9, and MkapC with Pkn8. These results indicate that there are complex PSTK networks in M. xanthus sharing common modulating factors.

A novel bacterial signalling system with a combination of a Ser/Thr kinase cascade and a His/Asp two-component system

Prokaryotes and eukaryotes have long been thought to use very different types of kinases (the His kinases of the 'bacterial' two-component systems versus the 'eukaryotic' Ser/Thr/Tyr kinases) to carry out signal transduction. This paradigm no longer holds true, because both systems are now found together in an increasing number of prokaryotic organisms and 'two-component' His kinase are present in eukaryotes. Pioneering work on bacterial protein serine threonine kinases (PSTKs) has been performed in Myxococcus xanthus, a soil bacterium with a complex life cycle that possesses orthologues of signalling-related kinases 'typical' of both the prokaryotic and the eukaryotic kingdoms. In the work reported in this volume of Molecular Microbiology, Nariya and Inouye describe a PSTK cascade that modulates the biochemical activity of MrpC, a CRP-like transcriptional regulator for essential developmental signalling pathways in M. xanthus whose transcription is under the control of a two-component system. This is the first report of both a functional PSTK cascade in bacteria and the use of both PSTK and two-component systems to control a single complex bacterial signalling event.

Diversity in domain architectures of Ser/Thr kinases and their homologues in prokaryotes

Background

Ser/Thr/Tyr kinases (STYKs) commonly found in eukaryotes have been recently reported in many bacterial species. Recent studies elucidating their cellular functions have established their roles in bacterial growth and development. However functions of a large number of bacterial STYKs still remain elusive. The organisation of domains in a large dataset of bacterial STYKs has been investigated here in order to recognise variety in domain combinations which determine functions of bacterial STYKs.

Results

Using sensitive sequence and profile search methods, domain organisation of over 600 STYKs from 125 prokaryotic genomes have been examined. Kinase catalytic domains of STYKs tethered to a wide range of enzymatic domains such as phosphatases, HSP70, peptidyl prolyl isomerases, pectin esterases and glycoproteases have been identified. Such distinct preferences for domain combinations are not known to be present in either the Histidine kinase or the eukaryotic STYK families. Domain organisation of STYKs specific to certain groups of bacteria has also been noted in the current anlaysis. For example, Hydrophobin like domains in Mycobacterial STYK and penicillin binding domains in few STYKs of Gram-positive organisms and FHA domains in cyanobacterial STYKs. Homologues of characterised substrates of prokaryotic STYKs have also been identified.

Conclusion

The domains and domain architectures of most of the bacterial STYKs identified are very different from the known domain organisation in STYKs of eukaryotes. This observation highlights distinct biological roles of bacterial STYKs compared to eukaryotic STYKs. Bacterial STYKs reveal high diversity in domain organisation. Some of the modular organisations conserved across diverse bacterial species suggests their central role in bacterial physiology. Unique domain architectures of few other groups of STYKs reveal recruitment of functions specific to the species.

Identification and gene expression analysis of a large family of transmembrane kinases related to the Gal/GalNAc lectin in Entamoeba histolytica

We identified in the Entamoeba histolytica genome a family of over 80 putative transmembrane kinases (TMKs). The TMK extracellular domains had significant similarity to the intermediate subunit (Igl) of the parasite Gal/GalNAc lectin. The closest homolog to the E. histolytica TMK kinase domain was a cytoplasmic dual-specificity kinase, SplA, from Dictyostelium discoideum. Sequence analysis of the TMK family demonstrated similarities to both serine/threonine and tyrosine kinases. TMK genes from each of six phylogenetic groups were expressed as mRNA in trophozoites, as assessed by spotted oligoarray and real-time PCR assays, suggesting nonredundant functions of the TMK groups for sensing and responding to extracellular stimuli. Additionally, we observed changes in the expression profile of the TMKs in continuous culture. Antisera produced against the conserved kinase domain identified proteins of the expected molecular masses of the expressed TMKs. Confocal microscopy with anti-TMK kinase antibodies revealed a focal distribution of the TMKs on the cytoplasmic face of the trophozoite plasma membrane. We conclude that E. histolytica expresses members of each subgroup of TMKs. The presence of multiple receptor kinases in the plasma membrane offers for the first time a potential explanation of the ability of the parasite to respond to the changing environment of the host.

Modulating factors for the Pkn4 kinase cascade in regulating 6-phosphofructokinase in Myxococcus xanthus

Myxococcus xanthus, a Gram-negative developmental bacterium, contains a large number of protein Ser/Thr kinases (PSTKs). Among these PSTKs, Pkn4 has been shown to be 6-phosphofructokinase (PFK) kinase. PFK associates with the regulatory domain of Pkn4 (Pkn4RD) and is activated by Pkn4-mediated phosphorylation. The activation of PFK is required to consume glycogen accumulated during early development and is essential for efficient sporulation. Using the yeast two-hybrid screen, we identified three new factors, MkapA, MkapB and MkapC, that interact with Pkn4 and each contains well-known protein-protein interaction domains. MkapB contains eight tandem repeats of the TPR (tetratrico peptide repeat) domain and its interaction with Pkn4RD was phosphorylation-dependent. MkapB remained associated with Pkn4RD. As a result, Pkn4 did not interact with PFK and its activation was inhibited. While deletion of the pfk-pkn4 operon did not inhibit fruiting body formation, the spore yield was low. In contrast, a mkapB deletion mutant exhibited a 24 h delay in fruiting body formation, accumulated less glycogen in the stationary phase and gave rise to 3.2% spore formation as opposed to 100% attained with DZF1. In addition to Pkn4, MkapA associated with other membrane-associated PSTKs, Pkn1, Pkn2, Pkn8 and Pkn9, while MkapB associated with Pkn8 and Pkn9, and MkapC with Pkn8. These results indicate that there are complex PSTK networks in M. xanthus that share common modulating factors.

An effective sporulation of Myxococcus xanthus requires glycogen consumption via Pkn4-activated 6-phosphofructokinase

6-Phosphofructokinase (PFK) is a key enzyme for glycolysis in both prokaryotes and eukaryotes. Previously, it was found that the activity of Myxococcus xanthus PFK increased 2.7-fold upon phosphorylation at Thr-226 by the Ser/Thr kinase Pkn4. The pkn4 gene is located 18 bp downstream of the pfk gene forming an operon, and both genes are expressed during vegetative growth and development. Here, we show that glycogen, which accumulates during stationary phase and early in development, is consumed during sporulation. A pfk-pkn4 deletion strain accumulated glycogen at a higher level than the wild-type strain, was unable to consume glycogen during developmental progression and exhibited a poor spore yield. From genetic complementation analysis of the pfk-pkn4 deletion strain with the pfk and pkn4 genes, it was found that glycogen consumption and a high spore yield require not only the pfk gene but also the pkn4 gene. Furthermore, phosphorylation is critical for glycogen consumption because the pfk gene engineered to express the mutant PFK (Thr-226-Ala) did not complement a pfk mutant. We propose that glycogen metabolism in M. xanthus is regulated in a similar manner to that in eukaryotes requiring a protein Ser/Thr kinase.

Role of two novel two-component regulatory systems in development and phosphatase expression in Myxococcus xanthus

We have cloned a two-component regulatory system (phoR2-phoP2) of Myxococcus xanthus while searching for genes that encode proteins with phosphatase activity, where phoR2 encodes the histidine kinase and phoP2 encodes the response regulator. A second system, phoR3-phoP3, was identified and isolated by using phoP2 as a probe. These two systems are quite similar, sharing identities along the full-length proteins of 52% on the histidine kinases and 64% on the response regulators. The predicted structures of both kinases suggest that they are anchored to the membrane, with the sensor domains being located in the periplasmic space and the kinase domains in the cytoplasm. The response regulators (PhoP2 and PhoP3) exhibit a helix-loop-helix motif typical of DNA-binding proteins in the effector domains located in the C-terminal region. Studies on two single-deletion mutants and one double-deletion mutant have revealed that these systems are involved in development. Mutant fruiting bodies are not well packed, originating loose and flat aggregates where some myxospores do not reshape properly, and they remain as elongated cells. These systems are also involved in the expression of Mg-independent acid and neutral phosphatases, which are expressed during development. The neutral phosphatase gene is especially dependent on PhoP3. Neither PhoP2 nor PhoP3 regulates the expression of alkaline phosphatases and the pph1 gene.

A central regulator of morphological differentiation in the multicellular bacterium Streptomyces coelicolor

In the multicellular bacterium Streptomyces coelicolor, functions of developmental (bald) genes are required for the biosynthesis of SapB, a hydrophobic peptidic morphogen that facilitates aerial hyphae formation. Here, we show that aerial hyphal growth and SapB biosynthesis could be activated independently from the normal developmental cascade by providing unprogrammed expression of functionally interactive genes within the ram cluster. ramC, ramS and ramR were essential for normal growth of aerial hyphae, and ramR, a response regulator gene, was a key activator of development. The ramR gene restored growth of aerial hyphae and SapB formation in all bald strains tested (albeit only weakly in the bldC mutant), many of which are characterized by physiological defects. Disruption of the ramR gene abolished SapB biosynthesis and severely delayed growth of aerial hyphae. Transcription of ramR was developmentally controlled, and RamR function in vivo depended on its putative phosphorylation site (D53). We identified and mapped RamR targets immediately upstream of the region encoding ramC and ramS, a putative operon. Overexpression of ramR in the wild-type strain increased SapB levels and caused a distinctive wrinkled surface topology. Based on these results, we propose that phenotypes of bald mutations reflect an early stage in the Streptomyces developmental programme similar to the spo0 mutations in the unicellular bacterium Bacillus subtilis, and that RamR has analogies to Spo0A, the Bacillus response regulator that integrates physiological signals before triggering endospore formation.

Characterization of a membrane-linked Ser/Thr protein kinase in Bacillus subtilis, implicated in developmental processes

PrkC was shown to be a eukaryotic-like (Hanks-type) protein kinase from Bacillus subtilis with a structural organization similar to that of the eukaryotic sensor Ser/Thr or Tyr kinases (e.g. the TGF beta or PDGF receptors). The molecule consists of a catalytic domain located in the cytoplasm, joined by a single transmembrane-spanning region (TMD) to a large extracellular domain. Using a genetic reporter system, involving the cI repressor of lambda, evidence was obtained indicating that PrkC forms a dimer, involving both the TMD and the external domain in dimerization. The purified catalytic domain of PrkC was shown to autophosphorylate and to phosphorylate an external target, MBP, in both cases on threonine. These two functions require the completely conserved K40 residue in subdomain II, which is essential for enzymatic activity. Importantly, both the mutant deleted for prkC and a K40R mutant exhibit decreased efficiency of sporulation and a significant reduction in biofilm formation, demonstrating that the catalytic activity of PrkC is necessary for these two developmental processes. In addition, we showed that the product of prpC, a PPM phosphatase encoded by the adjacent gene, co-transcribed with prkC, is also required for normal biofilm and spore formation.

Analysis of phosphorylation-dependent protein-protein interactions using a bacterial two-hybrid system

Phosphorylation-dependent protein-protein interactions provide the foundation for a multitude of intracellular signal transduction pathways. One of the goals of signal transduction research is to more precisely understand the nature of these phosphorylation-dependent interactions. Here, we describe a bacterial two-hybrid assay that allows for the rapid, efficient analysis of phosphorylation-dependent protein-protein interactions. In this system, the interacting protein domains are provided as fusion proteins in Escherichia coli. cells that contain a eukaryotic kinase. Specific phosphorylation of one of the fused protein domains results in a protein-protein interaction that can be detected as a change in the expression of a reporter gene. We also describe how this system can be modified to permit the use of cDNA libraries to identify either novel binding partners for a phosphorylated substrate or novel kinases that can induce a specific protein-protein interaction.

Evidence that a eukaryotic-type serine/threonine protein kinase from Mycobacterium tuberculosis regulates morphological changes associated with cell division

A eukaryotic-type protein serine/threonine kinase, PknA, was cloned from Mycobacterium tuberculosis strain H37Ra. Sequencing of the clone indicated 100% identity with the published pknA sequence of M. tuberculosis strain H37Rv. PknA fused to maltose-binding protein was expressed in Escherichia coli; it exhibited a molecular mass of approximately 97 kDa. The fusion protein was purified from the soluble fraction by affinity chromatography using amylose resin. In vitro kinase assays showed that the autophosphorylating ability of PknA is strictly magnesium/manganese-dependent, and sodium orthovanadate can inhibit this activity. Phosphoamino-acid analysis indicated that PknA phosphorylates at serine and threonine residues. PknA was also able to phosphorylate exogenous substrates, such as myelin basic protein and histone. A comparison of the nucleotide-derived amino-acid sequence of PknA with that of functionally characterized prokaryotic serine/threonine kinases indicated its possible involvement in cell division/differentiation. Protein--protein interaction studies revealed that PknA is capable of phosphorylating at least a approximately 56-kDa soluble protein from E. coli. Scanning electron microscopy showed that constitutive expression of this kinase resulted in elongation of E. coli cells, supporting its regulatory role in cell division.

Glycerol 3-phosphate inhibits swarming and aggregation of Myxococcus xanthus

We have cloned a gene of Myxococcus xanthus with similarities to the permease for glycerol 3-phosphate (G3P) of other bacteria. Expression of the gene increased significantly during the first hours of starvation. Swarming of the wild-type strain was inhibited and aggregation was delayed by G3P. Conversely, a DeltaglpT strain aggregated even on rich medium. These results indicate that G3P may function to regulate the timing of aggregation in M. xanthus.

An eukaryotic-type serine/threonine protein kinase involved in the carbon source-dependent pigment biosynthesis in Amycolatopsis mediterranei U32

The structural gene, pkmA, was cloned and sequenced from a rifamycin SV-producing Amycolatopsis mediterranei U32 strain. The N-terminal portion of the deduced amino acid sequence of pkmA showed significant similarity to the family of serine/threonine protein kinases. It contains all the structural features which are highly conserved in protein kinases, including the Gly-X-Gly-X-X-Gly motif of ATP binding and the essential amino acids known to be important for the recognition of the correct hydroxyamino acid in serine/threonine protein specific kinases. The protein possesses a region rich in Ala and Pro residues around the middle of pkmA open reading frame, which might be involved in the transmembrane function, as suggested by PhoA fusion protein analysis. The pkmA gene was expressed in Escherichia coli as a glutathione S-transferase (GST) fusion protein, and the protein was found to have the activity of autophosphorylation. A double crossover gene replacement was achieved by inserting an aparmycin resistance gene into pkmA in A. mediterranei chromosomal DNA. The phenotypic analysis of the mutant suggested that pkmA gene is involved in carbon source-dependent pigment formation in A. mediterranei U32.

Pph1 from Myxococcus xanthus is a protein phosphatase involved in vegetative growth and development

Myxococcus xanthus is a Gram-negative bacterium with a complex life cycle that includes vegetative swarming on rich medium and, upon starvation, aggregation to form fruiting bodies containing spores. Both of these behaviours require multiple Ser/Thr protein kinases. In this paper, we report the first Ser/Thr protein phosphatase gene, pph1, from M. xanthus. DNA sequence analysis of pph1 indicates that it encodes a protein of 254 residues (Mr = 28 308) with strong homology to eukaryotic PP2C phosphatases and that it belongs to a new group of bacterial protein phosphatases that are distinct from bacterial PP2C phosphatases such as RsbU, RsbX and SpoIIE. Recombinant His-tagged Pph1 was purified from Escherichia coli and shown to have Mn2+ or Mg2+ dependent, okadaic acid-resistant phosphatase activity on a synthetic phosphorylated peptide, RRA(pT)VA, indicating that Pph1 is a PP2C phosphatase. Pph1-expression was observed under both vegetative and developmental conditions, but peaked during early aggregation. A pph1 null mutant showed defects during late vegetative growth, swarming and glycerol spore formation. Under starvation-induced developmental conditions, the mutant showed reduced aggregation and failure to form fruiting bodies with viable spores. Using the yeast two-hybrid system, we have observed a strong interaction between Pph1 and the M. xanthus protein kinase Pkn5, a negative effector of development. These results suggest a functional link between a Pkn2-type protein kinase and a PP2C phosphatase.

A large family of eukaryotic-like protein Ser/Thr kinases of Myxococcus xanthus, a developmental bacterium

Myxococcus xanthus is a gram-negative bacterium that forms multicellular fruiting bodies upon starvation. Here, we demonstrate that it contains at least 13 eukaryotic-like protein Ser/Thr kinases (Pkn1 to Pkn13) individually having unique features. All contain the kinase domain of approximately 280 residues near the N-terminal end, which share highly conserved features in eukaryotic Ser/Thr kinases. The kinase domain is followed by a putative regulatory domain consisting of 185 to 692 residues. These regulatory domains share no significant sequence similarities. The C-terminal regions of 11 kinases contain at least 1 transmembrane domain, suggesting that they function as transmembrane sensor kinases. From the recent genomic analysis, protein Ser/Thr kinases were found in various pathogenic bacteria and coexist with protein His kinases. Phylogenetic analysis of these Ser/Thr kinases reveals that all bacterial Ser/Thr kinases were evolved from a common ancestral kinase together with eukaryotic Tyr and Ser/Thr kinases. Coexistence of both Ser/Thr and His kinases in some organisms may be significant in terms of functional differences between the two kinases. We argue that both kinases are essential for some bacteria to adapt optimally to severe environmental changes.

A eukaryotic-type protein kinase, SpkA, is required for normal motility of the unicellular Cyanobacterium synechocystis sp. strain PCC 6803

The genome of the unicellular cyanobacterium Synechocystis sp. strain PCC 6803 comprises many open reading frames (ORFs) which putatively encode eukaryotic-type protein kinase and protein phosphatase. Based on gene disruption analysis, a region of the hypothetical ORF sll1575, which retained a part of the protein kinase motif, was found to be required for normal motility in the original isolate of strain PCC 6803. Sequence determination revealed that in this strain sll1575 was part of a gene (designated spkA) which harbored an entire eukaryotic-type Ser/Thr protein kinase motif. Strain ATCC 27184 and a glucose-tolerant strain derived from the same isolate as the PCC strain had a frameshift mutation dividing spkA into ORFs sll1574 and sll1575. The structural integrity of spkA agreed well with the motility phenotype, determined by colony morphology on agar plates. The spkA gene was expressed in Escherichia coli as a His-tagged protein, which was purified by Ni2+ affinity chromatography. With [gamma-32P]ATP, SpkA was autophosphorylated and transferred the phosphate group to casein, myelin basic protein, and histone. SpkA also phosphorylated several proteins in the membrane fraction of Synechocystis cells. These results suggest that SpkA is a eukaryotic-type Ser/Thr protein kinase and regulates cellular motility via phosphorylation of the membrane proteins in Synechocystis.

Characterization of a two-component system, devR-devS, of Mycobacterium tuberculosis

By subtractive hybridization, we isolated genes, differentially expressed in virulent strain (dev), that are expressed at higher levels in the virulent Mycobacterium tuberculosis H37Rv strain in comparison to its avirulent counterpart, H37Ra, and consequently may be associated with the virulence phenotype of M. tuberculosis. A two-component system, devR-devS, was identified by DNA sequencing of a dev clone. DevR, the predicted gene product of devR, is a response regulator (RR) in the NarL/ UhpA subfamily of two-component systems. The devS gene product displayed homology with histidine protein kinases (HPKs) including UhpB, NarX and NarQ. The devR-devS locus is preceded by gene Rv3134c that encodes a putative alanine-aline- rich protein. This locus was conserved in M. tuberculosis and M. bovis BCG but not in other mycobacteria. A devR -lacZ transcription fusion demonstrated beta-galactosidase activity in M. smegmatis and in M. tuberculosis. The devR and devS genes were cotranscribed and the levels of their transcripts were lower in two isolates of the avirulent H37Ra strain in comparison to the virulent H37Rv strain of M. tuberculosis. The level of DevR protein was also lower in one of the H37Ra strains in comparison to the H37Rv strain. However, in a third isolate of H37Ra, RNA and protein expression was equivalent to that in the H37Rv strain. Electron microscopic immunogold analysis of M. tuberculosis grown in laboratory medium and within human monocytes revealed specific labelling for DevR protein within the bacteria and the phagosomal lumen of infected monocytes. These findings collectively suggest a potential role for devR-devS in the regulation of genetic programmes unique to the tubercle bacillus.

Magnitude of the CREB-dependent transcriptional response is determined by the strength of the interaction between the kinase-inducible domain of CREB and the KIX domain of CREB-binding protein

The activity of the transcription factor CREB is regulated by extracellular stimuli that result in its phosphorylation at a critical serine residue, Ser133. Phosphorylation of Ser133 is believed to promote CREB-dependent transcription by allowing CREB to interact with the transcriptional coactivator CREB-binding protein (CBP). Previous studies have established that the domain encompassing Ser133 on CREB, known as the kinase-inducible domain (KID), interacts specifically with a short domain in CBP termed the KIX domain and that this interaction depends on the phosphorylation of Ser133. In this study, we adapted a recently described Escherichia coli-based two-hybrid system for the examination of phosphorylation-dependent protein-protein interactions, and we used this system to study the kinase-induced interaction between the KID and the KIX domain. We identified residues of the KID and the KIX domain that are critical for their interaction as well as two pairs of oppositely charged residues that apparently interact at the KID-KIX interface. We then isolated a mutant form of the KIX domain that interacts more tightly with wild-type and mutant forms of the KID than does the wild-type KIX domain. We show that in the context of full-length CBP, the corresponding amino acid substitution resulted in an enhanced ability of CBP to stimulate CREB-dependent transcription in mammalian cells. Conversely, an amino acid substitution in the KIX domain that weakens its interaction with the KID resulted in a decreased ability of full-length CBP to stimulate CREB-dependent transcription. These findings demonstrate that the magnitude of CREB-dependent transcription in mammalian cells depends on the strength of the KID-KIX interaction and suggest that the level of transcription induced by coactivator-dependent transcriptional activators can be specified by the strength of the activator-coactivator interaction.

A novel transmembrane serine/threonine protein kinase gene from a rifamycin SV-producing amycolatopsis mediterranei U32

Genomic DNA sequencing in the vicinity of methylmalonyl-CoA mutase gene (mutAB) from a rifamycin SV-producing Amycolatopsis mediterranei U32 allowed us to clone, sequence, and identify a gene encoding a novel serine/threonine protein kinase (amk). The sequence contains a complete ORF of 1821 base pairs encoding a predicted protein of 606 amino acids in length. The N-terminal domain of the protein shows significant homology to the catalytic domain of other protein kinases from both prokaryotic and eukaryotic sources. It also contains all the structural features that are highly conserved in active protein kinases, including the Gly-X-Gly-X-X-Gly motif of ATP-binding and the essential amino acids known to be important for the recognition of the correct hydroxyamino acid in serine/threonine protein kinase. This protein kinase gene was expressed in Escherichia coli and was shown to have the ability of autophosphorylation. The autophosphorylated site was found to be the threonine at position 164 by labeled phosphoamino acid analysis and site-directed mutagenesis. The C-terminal half of protein kinase was found to contain strong transmembrane structures by PhoA fusion protein analysis, suggesting that Amk protein kinase is a transmembrane protein. A Southern hybridization experiment showed that this type of protein kinase is distributed ubiquitously and might play significant physiological roles in the various species of streptomycetes. However, overexpression of amk gene in Streptomyces cinnamonensis showed no effect on methylmalonyl-CoA mutase activity, monensin production and the hyphae morphology. Although its biological role is still unknown, Amk protein kinase is the first transmembrane serine/threonine protein kinase described for genus Amycolatopsis.

Antibiotic resistance as a stress response: complete sequencing of a large number of chromosomal loci in Staphylococcus aureus strain COL that impact on the expression of resistance to methicillin

Tn551 inactivation has identified several determinants--fem or auxiliary genes--that, in addition to the mecA gene, are also critical for the expression of high-level and homogeneous resistance to methicillin. Genetic and/or biochemical analysis has shown that of the nearly dozen aux mutations described so far most are in genes involved in cell wall synthesis (murE, pbp2, glmM, glnR, femA/B, llm, etc.) or in complex regulatory functions (sigmaB), suggesting that optimal expression of resistance may involve the cooperative functioning of a number of genes in cell wall metabolism as well as stress response. The exact mechanism of these functions is not known. In an attempt to explore this unusual aspect of methicillin resistance more fully, a Tn551 transposon library, constructed in the background of the highly and homogeneously methicillin-resistant Staphylococcus aureus strain COL, was screened for all independent insertional mutants in which the level of methicillin resistance of the parental strain (MIC, 1,600 microg/ml) was reduced by at least 15-fold and up to 500-fold. We now describe the sequencing of 21 Tn551-inactivated genes and their vicinities in 23 new auxiliary mutants that have been studied before. Using the inverted polymerase chain reaction (IPCR), we amplified fragments corresponding to the right and left junction of the Tn551 insertions, which were then sequenced by primer walking. The two largest groups of these new auxiliary genes encoded either proteins of unknown functions (6 genes) or showed homology with genes encoding proteins involved with putative sensory/regulatory activities (7 genes: protein kinases, ABC transporters, and a catabolite control protein). Sequencing upstream and downstream allowed the identification of a number of additional open reading frames, some of which may also include functions relevant for the expression of antibiotic resistance.

No longer an exclusive club: eukaryotic signalling domains in bacteria

Reversible phosphorylation of serine, threonine and tyrosine residues by the interplay of protein kinases and phosphatases plays a key role in regulating many different cellular processes in eukaryotic organisms. A diversity of control mechanisms exists to influence the activity of these enzymes and choreograph the correct concert of protein modifications to achieve distinct biological responses. Such enzymes and their adaptor molecules were long thought to be specific to eukaryotic cellular processes. However, there is increasing evidence that many prokaryotes achieve regulation of key components of cellular function through similar mechanisms.

A novel bacterial tyrosine kinase essential for cell division and differentiation

Protein kinases play central roles in the regulation of eukaryotic and prokaryotic cell growth, division, and differentiation. The Caulobacter crescentus divL gene encodes a novel bacterial tyrosine kinase essential for cell viability and division. Although the DivL protein is homologous to the ubiquitous bacterial histidine protein kinases (HPKs), it differs from previously studied members of this protein kinase family in that it contains a tyrosine residue (Tyr-550) in the conserved H-box instead of a histidine residue, which is the expected site of autophosphorylation. DivL is autophosphorylated on Tyr-550 in vitro, and this tyrosine residue is essential for cell viability and regulation of the cell division cycle. Purified DivL also catalyzes phosphorylation of CtrA and activates transcription in vitro of the cell cycle-regulated fliF promoter. Suppressor mutations in ctrA bypass the conditional cell division phenotype of cold-sensitive divL mutants, providing genetic evidence that DivL function in cell cycle and developmental regulation is mediated, at least in part, by the global response regulator CtrA. DivL is the only reported HPK homologue whose function has been shown to require autophosphorylation on a tyrosine, and, thus, it represents a new class of kinases within this superfamily of protein kinases.

Expression and characterization of the Mycobacterium tuberculosis serine/threonine protein kinase PknB

PknB is a member of the newly discovered eukaryotic-like protein serine/threonine kinase (PSTK) family of proteins. The pknB gene was cloned and expressed in Escherichia coli. The active recombinant protein was purified and shown to be reactive with antiphosphoserine antibodies, as well as with antibodies to the phosphorylated eukaryotic Ser/Thr kinases mitogen-activated protein kinase kinase 3 and 6, P38, and Creb. In vitro kinase assays demonstrated that PknB is a functional kinase that is autophosphorylated on serine/threonine residues and is also able to phosphorylate the peptide substrate myelin basic protein. Analysis of pknB expression in Mycobacterium tuberculosis indicates the presence of pknB mRNA in (i) organisms grown in vitro in bacteriological media, (ii) a murine macrophage in vitro infection model, and (iii) in vivo alveolar macrophages from a patient with tuberculosis.

Pkg2, a novel transmembrane protein Ser/Thr kinase of Streptomyces granaticolor

A 4.2-kb SphI-BamHI fragment of chromosomal DNA from Streptomyces granaticolor was cloned and shown to encode a protein with significant sequence similarity to the eukaryotic protein serine/threonine kinases. It consists of 701 amino acids and in the N-terminal part contains all conserved catalytic domains of protein kinases. The C-terminal domain of Pkg2 contains seven tandem repeats of 11 or 12 amino acids with similarity to the tryptophan-docking motif known to stabilize a symmetrical three-dimensional structure called a propeller structure. The pkg2 gene was overexpressed in Escherichia coli, and the gene product (Pkg2) has been found to be autophosphorylated at serine and threonine residues. The N- and C-terminal parts of Pkg2 are separated with a hydrophobic stretch of 21 amino acids which translocated a PhoA fusion protein into the periplasm. Thus, Pkg2 is the first transmembrane protein serine/threonine kinase described for streptomycetes. Replacement of the pkg2 gene by the spectinomycin resistance gene resulted in changes in the morphology of aerial hyphae.

Inhibition of development of Myxococcus xanthus by eukaryotic protein kinase inhibitors

Myxococcus xanthus is a social bacterium that lives in the soil and undergoes spectacular development to form multicellular fruiting bodies. It contains a large family of eukaryote-like serine/threonine protein kinases. We found that a number of inhibitors for eukaryotic protein serine, threonine, and tyrosine kinases could inhibit the development and sporulation of M. xanthus to various degrees. These results suggest that serine/threonine and tyrosine phosphorylation may be involved in development of M. xanthus. None of the inhibitors tested had any effect on vegetative growth of M. xanthus. Most of them seemed to act during the early stages of development. However, the expression of a very early development-specific gene, Omega4521, was not significantly affected by the inhibitors. The patterns of protein phosphorylation during development were also not significantly altered by the inhibitors, suggesting that the targets of the inhibitors are minor or unstable phosphoproteins but play key roles in fruiting-body formation in M. xanthus.

Novel families of putative protein kinases in bacteria and archaea: evolution of the "eukaryotic" protein kinase superfamily

The central role of serine/threonine and tyrosine protein kinases in signal transduction and cellular regulation in eukaryotes is well established and widely documented. Considerably less is known about the prevalence and role of these protein kinases in bacteria and archaea. In order to examine the evolutionary origins of the eukaryotic-type protein kinase (ePK) superfamily, we conducted an extensive analysis of the proteins encoded by the completely sequenced bacterial and archaeal genomes. We detected five distinct families of known and predicted putative protein kinases with representatives in bacteria and archaea that share a common ancestry with the eukaryotic protein kinases. Four of these protein families have not been identified previously as protein kinases. From the phylogenetic distribution of these families, we infer the existence of an ancestral protein kinase(s) prior to the divergence of eukaryotes, bacteria, and archaea.

Purification of two Bacillus subtilis proteins which cross-react with antibodies directed against eukaryotic protein kinase C, the His HPr kinase and trigger factor

As in eukaryotes, phosphorylation of Ser residues in proteins appears to be common phenomenon in bacteria. Surprisingly, however, very few Ser/Thr protein kinases have been identified and in this study antibodies directed against mammalian protein kinase C (PKC) have been used in attempts to isolate conserved Ser/Thr protein kinases. Using the mAb M7 against rat brain PKC, a single 70 kDa band was identified in total cell extracts of Bacillus subtilis by Western blotting after SDS-PAGE, whilst using polyclonal antibody alpha-PKC1p against Saccharomyces cerevisiae PKC a single 67 kDa band was identified by the same procedure. The two proteins were purified independently on the basis of antibody recognition employing two-dimensional gel electrophoresis as a final step, which allowed subsequent microsequencing. The 70 kDa band was thus identified as the phosphoenolpyruvate-dependent His HPr kinase, Enzyme 1 of the phosphotransferase system. This identity was confirmed using a mutant deleted for ptsl, encoding Enzyme 1. The 67 kDa protein was identified as a previously unknown B. subtilis 'trigger factor', homologous to an Escherichia coli protein-folding enzyme, peptidylprolyl cis-trans-isomerase implicated in cell division.

A serine/threonine protein kinase from Mycobacterium tuberculosis

Genomic DNA sequencing in the vicinity of the pstA-1 gene from Mycobacterium tuberculosis allowed us to clone, sequence and identify a gene encoding a 70-kDa protein. The size of the protein was confirmed by in vitro coupled transcription/translation. Its N-terminal domain shows extensive sequence similarity with the catalytic domain of eukaryotic serine/threonine protein kinases, and the protein was therefore called Mbk (mycobacterial protein kinase). The deduced amino acid sequence contains two transmembrane segments, which flank a highly repetitive region, suggesting a receptor-like anchoring. The mbk gene was overexpressed in Escherichia coli and the gene product (Mbk) was purified as a fusion protein with gluthatione S-transferase. Recombinant Mbk was found to be autophosphorylated on threonine residues and capable of phosphorylating myelin basic proteins from bovine brain and histones from calf thymus on serine residues, both in a manganese-dependent manner. The phosphorylation of myelin basic proteins by Mbk was inhibited by calcium and by staurosporine, a widely used inhibitor of eukaryotic protein serine/threonine kinases. A similar gene was found in Mycobacterium bovis BCG DNA by Southern blot analysis. Its expression was detected in cultures of M. bovis BCG by reverse transcriptase/PCR. Although its biological role is unknown, it is the first serine/threonine protein kinase characterized in Mycobacteria.

Positive-negative KG cassettes for construction of multi-gene deletions using a single drug marker

Positive-negative KG cassettes were developed in order to create a number of independent deletion mutations on the bacterial chromosome using a single drug marker. These cassettes consist of a kanamycin-resistant (KmR) gene for positive screening and a galactokinase gene (galK) for negative screening. Both genes are in an operon driven by the native KmR promoter and are flanked by identical fragments of yeast chromosomal DNA approximately one kb in size. An internal region of a cloned target gene of a bacterium is replaced with a cassette, which is then transformed into the bacterium. The intact gene on the chromosome is replaced with the mutated gene by homologous recombination. From the KmR cells thus obtained, those cells which lose both KmR and galK genes by homologous recombination between the identical yeast DNA fragments are subsequently screened on plates containing 2-deoxygalactose, a non-metabolizable analogue of galactose. This method was applied to isolate a triple-deletion mutant of pkn3, pkn1, and pkn11 from Myxococcus xanthus.

Stigmatella aurantiaca fruiting body formation is dependent on the fbfA gene encoding a polypeptide homologous to chitin synthases

Stigmatella aurantiaca is a prokaryotic organism that undergoes a multicellular cycle of development resulting in the formation of a fruiting body. For analyzing this process, mutants defective in fruiting body formation have been induced by transposon mutagenesis using a Tn5-derived transposon. About 800 bp upstream of the transposon insertion of mutant AP182 which inactivates a gene (fbfB) involved in fruiting, a further gene (fbfA) needed for fruiting body formation was detected. Inactivation of fbfA leads to mutants which form only non-structured clumps instead of the wild-type fruiting body. The mutant phenotype of fbfA mutants can be partially suppressed by mixing the mutant cells with cells of some independent mutants defective in fruiting body formation. The fbfA gene is transcribed after 8 h of development as determined by measuring the induction of beta-galactosidase activity of a fbfA-delta(trp)-lacZ fusion gene and by Northern (RNA) analysis using an insertion encoding a stable mRNA. The predicted polypeptide FbfA shows a homology of about 30% to NodC of rhizobia, an N-acetylglucosamine-transferase which is involved in the synthesis of the sugar backbone of lipo-oligosaccharides. These induce the formation of the root nodules in the Papilionaceae. Besides the predicted molecular mass of 45.5 kDa, the hydropathy profile reveals a structural relationship to the NodC polypeptide.

Effects of overexpression of Pkn2, a transmembrane protein serine/threonine kinase, on development of Myxococcus xanthus

Pkn2 is a putative transmembrane protein serine/threonine kinase required for normal development of Myxococcus xanthus. The effect of Pkn2 overexpression on development of M. xanthus was examined by expressing pkn2 under the control of a kanamycin promoter. Pkn2 was clearly detected by Western blot (immunoblot) analysis in the overexpression strain (the PKm/pkn2 strain) but could not be detected in the wild-type strain. Overexpressed Pkn2 was located almost exclusively in the membrane fraction, suggesting that Pkn2 is a transmembrane receptor-type protein Ser/Thr kinase. The PKm/pkn2 strain formed fruiting bodies more slowly than the wild-type strain, in contrast to a Pkn2 deletion strain, the delta pkn2 strain, which developed faster than the wild-type strain. However, spore production was reduced in both the PKm/pkn2 and delta pkn2 strains. These data suggest that Pkn2 functions as a negative regulator for fruiting-body formation and that the proper level of Pkn2 is necessary for maximum myxospore yield.

Bacterial signalling involving eukaryotic-type protein kinases

Protein Ser, Thr and Tyr kinases play essential roles in signal transduction in organisms ranging from yeast to mammals, where they regulate a variety of cellular activities. During the last few years, a number of genes that encode eukaryotic-type protein kinases have also been identified in four different bacterial species, suggesting that such enzymes are also widespread in prokaryotes. Although many of them have yet to be fully characterized, several studies indicate that eukaryotic-type protein kinases play important roles in regulating cellular activities of these bacteria, such as cell differentiation, pathogenicity and secondary metabolism. A model based on the possible coupling between two-component systems and eukaryotic-type protein kinases is proposed to explain the function of eukaryotic-type protein kinases in bacterial signalling in the light of studies in bacteria, as well as in plants and yeast. These two groups of eukaryotes possess signal-transduction pathways involving both two-component systems and eukaryotic protein kinases.

Reciprocal regulation of the differentiation of Myxococcus xanthus by Pkn5 and Pkn6, eukaryotic-like Ser/Thr protein kinases

Myxococcus xanthus contains a large family of genes encoding eukaryotic-like serine/threonine kinases. Among them, two genes, pkn5 and pkn6, are divergently located on the chromosome and share a 46 bp promoter region between their transcription initiation sites, as determined by RNA protection. Pkn5, consisting of 380 amino acid residues, is a soluble protein in the cytoplasm, while Pkn6, consisting of 710 amino acid residues, is a transmembrane protein. Its membrane topology was determined using the Pkn6-PhoA fusion protein in Escherichia coli, which has a single transmembrane domain with the N-terminal domain in the cytoplasm and the C-terminal domain outside the cytoplasmic membrane. Both proteins, when expressed in E. coli, were autophosphorylated: Pkn5 only at Ser, and Pkn6 at both Ser and Thr. In M. xanthus, both genes are expressed constitutively throughout the life cycle, with slight increases at an early stage of development. Most strikingly, a pkn5-deletion strain forms fruiting bodies much faster than the wild-type strain, while a pkn6-deletion strain develops slower than the wild-type strain. These results, together with the fact that the pkn5-deletion strain is able to form fruiting bodies on semi-rich media, suggest that Pkn5 and Pkn6 have reciprocal roles in M. xanthus growth and development. Furthermore, Pkn6 may be a transmembrane sensor of external signals for development, while Pkn5 is a kinase that negatively regulates M. xanthus development.

MlpA, a lipoprotein required for normal development of Myxococcus xanthus

The mlpA gene encoding a 236-residue polypeptide has been identified immediately downstream of the oar gene of Myxococcus xanthus (M. Martinez-Canamero, J. Munoz-Dorado, E. Farez-Vidal, M. Inouye, and S. Inouye, J. Bacteriol. 175:4756-4763, 1993). The amino-terminal 21 residues of MlpA encode a typical prokaryotic signal sequence with a putative lipoprotein cleavage site. When expressed in Escherichia coli in the presence of [2-3H]glycerol, 3H-labeled MlpA had a molecular mass of 33 kDa and was found to be associated with the membrane fraction. Globomycin, an inhibitor of signal peptidase II, caused a shift in the mobility of E. coli-expressed MlpA to 35 kDa. Subsequently, a mlpA disruption strain (oar+) was constructed and found to have delayed fruiting body formation (by approximately 36 h), with significantly larger fruiting bodies being produced compared with those of the wild-type strain. Nevertheless, spore yields for the two strains were identical after 120 h of development. These data indicate that MlpA, the lipoprotein identified in M. xanthus, is required for normal fruiting body formation.